Drive circuits have long been used to amplify and transmit a signal from an input source to a load and associated circuitry. One application of such drive circuitry is in a thermal printer connected between a logic signal source and a thermal print element. For driving a thermal printer, drive circuits usually drive an output power transistor that, in turn, drives a thermal print wafer.
Although previously available drive circuits have been generally satisfactory for use in printers, a number of problems have been associated with such conventional drive circuits. Often, the drive circuitry included no provisions for protecting the output power transistor or the load during open circuit or floating conditions at the drive circuit input. Such input conditions often resulted in leakage currents in the drive circuit that caused a logic "1" or "on" condition at the drive circuit output. Responding to the logic "1" condition, the output transistor would turn "on" and a steady state current flowed through the thermal print wafer. Since neither the output transistor nor the wafer were usually designed for a steady state "on" operation, one or both of these elements would be destroyed.
Furthermore, conventional drive circuits have, in the past, produced undue power dissipation through the use of a drain resistor on the output transistor of the drive circuit. In many conventional drive circuits, it was necessary to hand match transistors to gain desired performance characteristics. Hand matching is an expensive and time consuming operation. Also, heretofore available drive circuits have been expensive, relatively unreliable and difficult to build because of the large number of chips and wire bonds necessary.